Until fairly recently, the preferred, indeed the only means by which to display information in the electronic medium was to use a video monitor comprising a cathode ray tube ("CRT"). CRT technology has been well known for over 50 years, and has gained widespread commercial acceptance in applications ranging from desktop computer modules to home televisions and industrial applications. CRTs are essentially large vacuum tubes having one substantially planar surface upon which information is displayed. Coated on the inside of the CRT planar surface is a layer of phosphors which respond by emitting light when struck by electrons emitted from the electron gun of the CRT. The electron gun is disposed in an elongated portion which extends away from the inside of the CRT display surface.
While CRTs are widely used in numerous applications, there are several inherent limitations to the application of CRT technology. For example, CRTs are relatively large and consume a great deal of energy. Moreover, as they are fabricated of glass, the larger the display surface, the heavier the CRT. Given the need for the electron gun to be spacedly disposed from the phosphor surface of the display, CRTs have a substantial depth dimension. Accordingly, CRTs have little practical applicability in small and portable applications, such as handheld televisions, laptop computers, and other portable electronic devices which require the use of displays.
To answer the needs of the marketplace for smaller, lighter, more portable display devices, manufacturers have created numerous types of flat panel display devices. Examples of flat panel display devices include active matrix liquid crystal displays (AMLCD's), plasma displays, and electroluminescent displays. Each of these types of displays has use for a particular market application, though each are accompanied by various limitations which make them less than ideal for certain applications. Principal limitations inherent in devices such as AMLCD's relate to the fact that they are fabricated predominantly of inorganic semiconductor materials by semiconductor fabrication processes. These materials and processes are extremely expensive, and, due to the complexity of the manufacturing process, cannot be reliably manufactured in high yields. Accordingly, the costs of these devices are very high with no promise of immediate cost reduction.
One preferred type of device which is currently receiving substantial research effort is the organic electroluminescent device. Organic electroluminescent devices ("OED") are generally composed of three layers of organic molecules sandwiched between transparent, conductive and/or metallic conductive electrodes. The three layers include an electron transporting layer, an emissive layer, and a hole transporting layer. As used herein, the charge carriers which combine in the emissive layer are electrons and holes. The electrons are negatively charged atomic particles and holes are positively charged counterparts. OED's are attractive owing to low driving voltage, i.e., less than about 20 volts. Hence, they have a potential application to full color flat emissive displays.
One problem which has plagued OED's has been the stability of the devices, and in particularly the thermal stability of the hole transporting material. Degradation in these materials have been observed and are typically attributable to one of two problems: (1) phase stability of hole transporting materials, namely the tendency to recrystallize or aggregate, which is a function of the glass transition temperature of the hole transporting material; and (2) the adhesion of hole transporting materials which are typically organic, to the materials from which the electrodes are fabricated, which are typically inorganic. These problems may be due to thermal expansion coefficient mismatch and surface tension mismatch (hydrophobicity or hydrophilicity) between the organic hole transporting material and the inorganic electrode materials.
To address issues arising from this mismatch between the organic and inorganic layers, numerous solutions have been proposed. For example, in U.S. Pat. No. 4,356,429, it was proposed to insert a thin layer of organometallic material between the organic and inorganic materials. The organometallic materials have physical properties which are between those of the organic and inorganic materials and thus serve as a buffer layer. Examples of such organometallic materials include metal phthalocyanine complexes such as copper phthalocyanine, and zinc phthalocyanine. Another solution was proposed in an article authored by J. Sheats, et al in Science Magazine (Science 273, 884, (1996)) which describes spincoating a thin layer of polyaniline between the organic and inorganic materials. The article reports that the lifetime of the OED was drastically increased. One final approach which may be employed is described in commonly assigned, copending patent application Ser. No. 08/743,257, entitled "Organic Electroluminescent Light Emitting Material and Device Using Same", filed 4 Nov. 1996 which is now U.S. Pat. No. 5,747,183 in the names of Song Shi, et al, the disclosure of which is incorporated herein by reference. In that patent application, the inventors describe a structure in which a layer of a conductive polymer passivating material is disposed between the electrode structure and the organic materials of the organic electroluminescent display device.
While each of these approaches provides a solution to the problem, there are numerous shortcomings attendant to each. For example, organometallic materials described in the '429 patent are both relatively expensive and difficult to deposit with high reliability. Similarly, spincoat deposition of polyaniline between the hole transporting and ITO materials is a relatively difficult process which may both increase the cost and decrease the manufacturing yield of OED's.
Accordingly, there exists a need for improved electrode structures for use in OED's. The electrode structures should enhance the thermal coefficient matching and the surface tension matching between the organic hole transporting layer materials and the inorganic electrode materials. The solution to the problem should be fabricated of a relatively inexpensive material which can be deposited via readily available, highly-rel reliable, inexpensive processes.